1. Field of the Invention
The present invention relates to a glow plug used to assist in starting a diesel engine and to a method of manufacturing the same.
2. Description of Related Art
As an example glow plug used to assist in starting a diesel engine, a glow plug using a sheath heater is known. The sheath heater is configured as follows: a heat-generating coil is accommodated within a bottomed sheath tube having a closed front end, and a magnesia powder serving as an insulating powder is charged into the sheath tube so as to electrically insulate the heat-generating coil and the sheath tube from each other. The sheath heater is held in an axial bore of a tubular metallic shell such that its front portion projects from the metallic shell, whereas its rear portion is surrounded by a wall of the axial bore. The metallic shell and the sheath tube are electrically connected to each other. One end of the heat-generating coil is electrically connected to an inner surface of the sheath tube, and the other end of the heat-generating coil is electrically connected to one end of an axial rod, which is inserted into the axial bore of the tubular metallic shell while being electrically insulated from the metallic shell. When electricity is conducted between the metallic shell and the other end of the axial rod exposed from a rear end of the metallic shell, the heat-generating coil generates heat.
In the process of manufacturing such a glow plug, in order to seal a magnesia powder which is charged into the sheath tube as mentioned above, a seal member (elastic packing) formed of heat-resistant silicone rubber, fluorine-containing rubber, or the like is fitted into a rear end portion of the sheath tube. Subsequently, swaging or a like process is carried out on the sheath tube so as to diameter-reduce at least the rear end portion of the sheath tube, whereby an outer circumferential surface of the seal member and an inner circumferential surface of the sheath tube come into close contact with each other, thereby establishing a sealed condition (refer to, for example, Japanese Patent Application Laid-Open No. 2003-17230).
However, when the amount of the magnesia powder charged into the sheath tube is excessively large, or when the sheath tube is influenced by vibration generated in the course of swaging, the magnesia powder may intrude into a region between the inner circumferential surface of the sheath tube and the outer circumferential surface of the seal member. This may cause moisture in an ambient atmosphere to enter the sheath tube via the intruding magnesia powder. When the entry of moisture induces generation of gas from heat of the heat-generating coil, there is risk of deforming the sheath tube and rendering the heat-generating coil fragile.